This invention relates to a self-starting brushless electric motor of the type which comprises reluctance poles (ferromagnetic salient poles) at least on one of the two relatively moving motor parts and one or more permanent-magnetic poles in the pole system.
Self-starting brushless electric motors can be supplied with direct current pulses of a single polarity or with alternating current polarity. When motors with moderate shaft power are supplied electronically, direct-current pulse supply uses the least number of electronic switches and thus gives the lowest system costs for motor and supply electronics. On the other hand, for higher power, when the number of electronic switches in the supply electronics in the motor must anyway be increased, it may be advantageous to supply the motor with alternating current polarity so that electric power will be supplied to the motor during both half-periods, thus achieving more uniform torque development and reducing the electrical conduction losses in the winding.
A single-strand brushless motor has only one winding supplied from a single external current source and provided on one of two or more parts which are rotatable or otherwise movable in relation to each other. Such a motor can be self-starting, i.e. develop a driving torque when at standstill in a predetermined direction, the preferential starting direction, only if this starting direction is inherent in the design of the motor. Self-starting in the preferential direction may be built into the motor by providing asymmetry in the soft-magnetic iron core, e.g. through the use of asymmetrical salient poles and/or asymmetrical permanent-magnet poles, or by providing auxiliary windings with no connection to external current sources, e.g. short-circuited current paths as in known shaded-pole motors. Such current paths can only conduct current under the influence of a varying magnetic field linked to these current paths. In order for current to flow in such current paths when the motor is stationary, the winding connected to an external current source must be supplied with a pulsed or alternating current.
It can be shown theoretically that motors that are not provided with auxiliary windings but are nevertheless able to exert torque in any rotor position even when the motor winding is deenergized must always contain a permanent-magnet pole.
In the following the description is limited to motors for rotary movement which have a first part, in the following called the stator, provided with a winding, and a second part, in the following called the rotor, arranged inside the stator and rotatable in relation thereto. It will, however, be appreciated that these two parts may exchange places, that the air gap separating the stator from the rotor need not be cylindrical but may equally well be flat or conical, and that the relative movement between the parts of the motor need not be rotary but may equally well be linear or a combination of rotary and linear, i.e. occur simultaneously about and along an axis of rotation.
The function of the motor may be described as comprising work cycles which are repeated a given number of times for each revolution. At extremely low speed, e.g. when starting up from standstill, the work cycle for a motor designed to be supplied with DC pulses of one polarity consists of one part when the winding carries current and another part when the winding is currentless. For a motor designed to be supplied with current pulses of alternating polarity the work cycle consists of one part when the winding is supplied with current of one polarity, followed by a currentless part and thereafter a part when the winding is supplied with current of opposite polarity followed by another currentless part of the work cycle.
In the currentless state the rotor must reach a starting position, i.e. a position in which the winding, if supplied with current, gives rise to a driving torque, namely a torque in the preferential direction of the motor, that is sufficiently high to overcome any frictional torque or the like in the motor and/or in the object driven by the motor. The torque generated in the motor through permanent-magnetic forces must maintain its direction and be of sufficient strength until the rotor reaches a position in which the winding can be energized. It will be understood that the demand for torque development in currentless state means that the motor must include at least one permanent-magnet pole.
Motors operating in accordance with the principles described and exhibiting magnetic asymmetry in the pole system are known through WO90/02437 and WO92/12567. An object of the present invention is to obtain improvements in motors of the type represented by the motors in the aforesaid publications.
This object is achieved by means of the arrangement of magnetically active stator and rotor elements (poles).
Besides the opportunity of realizing constructionally alternative embodiments, the invention also offers the opportunity of increasing the force generated by the motorxe2x80x94torque in a rotating motor and linearly acting force or xe2x80x9ctractive forcexe2x80x9d in a linear motorxe2x80x94in one or more respects:
Increasing the torque generated by permanent-magnet poles that pulls the rotor of the currentless motor to the nearest starting position. Such improvement is advantageous in applications where high frictional torque may appear in the driven object, for example in shaft seals.
Increasing torque appearing in a motor whose rotor is stationary in a starting position and whose winding is supplied with the highest current available. Such improvement is also advantageous in the situations mentioned in the preceding paragraph.
Increasing, at least in certain embodiments, the air-gap power of the motor for given heat losses, thereby giving a smaller and economically more favourable motor for a given purpose, which may be a great advantage when low motor weight is of importance for certain types of applications, e.g. in hand-held tools or other hand-held objects, but is also an economic advantage in general, provided an unavoidable cost increase in the supply electronics does not cancel the effect.
The magnetically active elements in the motor of relevance to the invention are as follows:
Coils on the stator
In principle the coils form a single current circuit and may be connected in series and/or in parallel. When the supply electronics consist of several units operating in parallel, these may be connected each to its own coil or group of coils, as if they formed a single electrical circuit. Instead of supplying the winding with alternating current polarity a two-part winding can be used, the two winding halves being supplied with a single current polarity, but the winding halves having magnetically opposite directions.
Ferromagnetic salient poles (reluctance poles)
In most of the motors shown according to the invention, ferromagnetic salient poles, in the following also called reluctance poles, are to be found on the stator, alone or together with permanent-magnet poles.
There may also be reluctance poles on the rotor, but preferably not mixed with permanent-magnet poles. A mixture of these pole types on the rotor can be contemplated but is normally not meaningful.
The reluctance poles on both stator and rotor may be magnetically asymmetrical. For magnetically asymmetrical stator poles the asymmetry should be directed in the opposite direction to the preferential direction of motion of the motor, whereas on the rotor the asymmetry should be in the same direction as the preferential direction of motion.
Alternatively or in addition, the reluctance poles on both stator and rotor may, however, show a certain magnetic asymmetry in the opposite direction to that described above without this making the motor inoperable.
Permanent-magnet poles
Motors with only reluctance poles on the rotor must always be provided with a permanent-magnet pole on the stator. The permanent-magnet poles on the stator preferably are magnetically balanced, i.e. equal in number and size of both polarities.
In certain cases it is an advantage if the permanent-magnet poles are asymmetrical.
Motors with permanent-magnet rotor poles designed to be supplied with current pulses of a single polarity must always be provided with a permanent-magnet pole on the stator. If such permanent-magnet poles are asymmetrical in shape and have a main pole part and an auxiliary pole part, their main pole part may advantageously be displaced in the direction opposite to that of the auxiliary pole part, e.g. from a position it would have if the pole were symmetrical, consisting only of a main pole part.
Motors with permanent-magnet rotor poles may lack permanent-magnet poles on the stator. Such motors are self-starting only if they are supplied with current pulses of alternating polarity. Such motors will then have more uniform torque development and higher average torque for given winding losses than the motors supplied with current pulses of a single polarity.
The permanent-magnet poles, both symmetrical and asymmetrical, may have skewed ends or edges, i.e. edges running at an angle to the direction of the rotor axis. In some cases such skewing of the edges of the permanent-magnet poles may be extremely beneficial to the function of the motor. Such skewed edges need not be embodied in geometric shapes. It is sufficient for the edges to consist of demarcation lines (demarcation zones) relating to the imprinted magnetic polarisation (in, for example, a permanent-magnet pole), i.e., they are imprinted when the permanent-magnet poles are magnetized.
These demarcation lines for zones with the same magnetic polarization may run other than linearly without the function of the motor being greatly affected.
The magnetic asymmetry can be achieved in several ways within the scope of the invention and the appended claims and some of them will be explained below.
As in the prior art motors, the magnetic asymmetry aims at building the preferential starting direction into the motor, but the magnetic asymmetry in motors according to the present invention also serves other purposes.
Basically, an additional purpose of the magnetic asymmetry as utilized in the present invention is to extend what is herein termed the pull-in distance. This is the distance over which a pole, a permanent-magnet pole or a magnetized reluctance pole, on one of the motor parts is capable of attracting a pole on the other motor part sufficiently to cause the two poles to be pulled towards one another from a first stable position, such as the indrawn position, to the next stable position, such as the starting position, in which they are mutually aligned magnetically and, accordingly, no magnetic pull force in the direction of relative movement exists between the poles (only a magnetic pull in a direction transverse to that direction).
During this pull-in motion the permeance between the two poles or, in other words, the magnetic flux passing between them (assuming that the magnetomotive force is constant) should increase steadily to a maximum value occurring when the poles are magnetically aligned. An extension of the pull-in distance thus calls for a lowering of the mean value of the rate of flux change over the pull-in distance.
Such a lowering can be accomplished by means of magnetic asymmetry, e.g. by providing on at least one of the poles an additional pole part extending in the relative preferential starting direction so that the pole will have a main pole part and an auxiliary pole part which determines the preferential starting direction.
In the starting position and the indrawn position, the auxiliary pole part extends at least to a point in the vicinity of the next pole (as seen in the relative preferential starting direction) on the other motor part and it may even slightly overlap that pole. However, an overlapping portion of the auxiliary pole part must not carry as much flux per unit length of overlap (measured circumferentially) as overlapping portions of main pole parts.
Assuming that in a rotary motor chosen by way of example both the leading ends and the trailing ends of both the stator poles and the rotor poles extend axially, magnetic asymmetry of a stator pole could in most cases in principle be observed in the following way. The rotor of the motor is replaced with a homogenous ferromagnetic cylinder of the same diameter as the rotor and the flux density in the air gap is measured along an axially extending line on the cylinder surface as the cylinder is rotated to move the line in the preferential direction of rotation past the pole. A graph showing the measured flux density (as averaged over the length of the line) versus the angular position of the line relative to the pole would rise, more or less steadily or in more or less distinct steps, from a point near zero at the leading end of the pole, to a roughly constant value under the main portion of the pole and then decline steeply at the trailing end. If the pole were magnetically symmetric instead, the graph would by symmetrical and resemble a Gaussian curve.
With suitable modifications the above-described principle is applicable also in other cases, such as when observing magnetic asymmetry of a rotor pole or a pole whose leading and trailing ends do not extend axially. For example, where the ends of the pole are skewed so that they extend along a helical line, the observation can be made with the measurement of the flux density taking place along a correspondingly skewed line.
In the case of permanent-magnet poles with uniform radial dimension and uniform radial magnetic polarization, magnetic pole asymmetry can result from the pole shape. For example, the leading and trailing ends of the pole may have different lengths in the axial direction of the motor. A similar effect can also be achieved by magnetically imprinting poles with a corresponding shape in a ring of permanent-magnetic material of uniform thickness. In this case the shape of the permanent-magnet ring has nothing to do with the magnetic pattern or xe2x80x9cmagnetic shapexe2x80x9d.
Magnetic pole asymmetry can also be achieved by providing a permanent-magnet pole with different radial dimensions at the leading and trailing ends, respectively, (i.e. by giving the air gap at the pole a width that varies in the direction of the relative movement of the rotor parts) but giving it a uniformly strong magnetization over its entire volume.
Several methods can of course be used simultaneously in order to achieve magnetic asymmetry for the permanent-magnet poles.
There are also several ways of achieving magnetic asymmetry for salient ferromagnetic poles, the reluctance poles. One method is to arrange the surface of such a pole facing the air gap asymmetrically with regard to its extension in the axial direction of the motor, in which case the entire pole surface may be situated at the same radial distance from the axis of rotation.
Another method is to make the projection surface of the reluctance pole (the surface facing the air gap) symmetrical, but vary its radial distance from the axis of rotation, i.e. vary the width of the air gap along the pole surface, stepwise or continuously, in relation to an imagined (cylindrical) surface on the other motor part.
A third method is to vary the magnetic saturation flux density along the pole surface. This can be achieved by using different magnetic materials for different parts of the salient pole, or it can be achieved by varying the filling factor of the laminated ferromagnetic poles, or by means of punched recesses, for example, below the actual pole surface (so that the actual pole surface appears to be homogenous), or by varying the radial dimension of an auxiliary pole part such that it will have a shape resembling the profile of the curved beak of a bird.
Of course several methods of achieving magnetic asymmetry can be used simultaneously. The choice of how to achieve asymmetry is usually dependent on a balance between the manufacturing costs of the actual motor and the cost of the supply electronics, since the choice of the type of asymmetry may affect the size of the power electronic switch elements included in the supply electronics.
As will become apparent, in motors embodying the invention magnetic asymmetry may characterize not only an individual pole of a group of poles which are associated with a common winding coil such that all poles of the group are subjected to the magnetic field produced upon energization of the coil. It may also characterize the pole group and then not only by virtue of magnetic asymmetry of one or more individual poles but also by virtue of an asymmetrical positioning of an individual pole within a pole group or on the rotor.
A pole of a pole group on the stator is asymmetrically positioned if a rotor pole is moved through a distance longer or shorter than one-half rotor pole pitch when it is moved between a position in which it is magnetically aligned with that stator pole and the next adjacent position in which any pole on the rotor is magnetically aligned with a stator pole of a different pole type or, in the case of a stator having only permanent magnets, a pole of different polarity.
In other words, a permanent-magnet pole, for example, on the stator is asymmetrically positioned with respect to a reluctance pole in the same or a different pole group if a rotor pole traverses a distance which is longer or shorter than one-half rotor pole pitch when the rotor moves between a position in which a rotor pole is magnetically aligned with that permanent-magnet pole, i.e. is in the starting position, to the next following or next preceding position in which a rotor polexe2x80x94which may be any rotor polexe2x80x94is in an indrawn position.
In a corresponding manner, magnetic asymmetry resulting from asymmetric positioning of poles may also exist in the rotor. For example, in pole row on a rotor comprising permanent-magnet poles of alternating polarity, the North-pole permanent-magnet poles may be displaced in either direction from a central position between the South-pole permanent-magnet poles with all like poles substantially equally spaced.
It should be noted that in the context of the present invention a pole group (pole unit) may comprise a single pole or a plurality of poles associated with a magnetizing coil.